1. Field of the Invention
The present invention relates to a pushbutton switch key sheet for use in an operating portion of various electric/electronic apparatuses, such as a mobile phone, a PDA, a car navigation apparatus, and a car audio apparatus, and more particularly, to a key sheet suitable for use in a structure in which one or two or more key tops are exposed through one operational opening provided in the casing of an apparatus.
2. Description of the Related Art
A pushbutton switch is known in which, as in a case of a mobile phone (101) shown in FIG. 14, due to a demand for a reduction in the size of the operating portion and from the viewpoint of artistic design property, a plurality of key tops (103) of a key sheet (102) are exposed in a clustered state through a single operational opening (101b) formed in a casing (101a). In the key sheet (102), a base sheet (104) for retaining the key tops (103) is formed solely of a flexible rubber-like elastic material, such as silicone rubber, so when, during use, the mobile phone (101) is, for example, set upright or laid face down as shown in FIG. 15, the base sheet (104) may suffer overall distortion due to the weight of the key tops (103). Then, a positional deviation may occur between pushers (104a) of the base sheet (104) and metal belleville springs (105a) provided on a board (105), resulting in a faulty operation state in which it is impossible to effect input if the key tops (103) are depressed.
To solve this problem, the present applicant has proposed, for example, in the document of JP 2004-319396 A, a key sheet (112) as shown in FIG. 16, and in the document of JP 2004-362891 A, a key sheet (122) as shown in FIG. 17. Those key sheets (112, 122) are equipped with a base sheet (114, 124) whose base portion (114a, 124a) formed of a rubber-like elastic material is reinforced by a reinforcing portion (114b, 124b) composed of a reinforcing member, such as a resin film or a hard resin plate, so even if the mobile phone (101) is laid face down, no distortion of the key sheet (112, 122) is generated.
Thus, in the key sheet (112, 122) equipped with the reinforcing portion (114b, 124b), it is possible to prevent generation of overall distortion of the key sheet (112, 122). However, as in a case of so-called oblique depression, in which a key top (113, 123) is depressed obliquely downwards, when a sidewise shifting force is applied to the key top (113, 123) of, for example, the key sheet (122), the key top (123) may be greatly tottered sidewise as shown in FIG. 18, or key tops (123) may overlap as shown in FIG. 19.